Add ST_FurthestPoint

For now, if the furthest point is not unique, we return the first one we
find. The return type is POINT.
Make ST_FurthestPoint return ALL furthest pointS.

Now if the furthest point is not unique, we return all furthest points
in a MULTIPOINT. If it is unique, it is returned as a POINT. (In the
java code, the return type is `Puntal`.)